1. Field of the Invention
This invention relates to an optical recording medium of phase change type and a method for preparing such an optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information. Optical recording media of the phase change type are of great interest since they can be directly overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Ge-Te systems which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites.
Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms with the crystallization of the recording layer.
In the conventional optical recording medium of phase change type, the recording layer has been formed by such means as vacuum deposition apparatus, and the recording layer immediately after its formation is in non-crystalline state. When the disc having such recording layer is utilized for a rewritable medium, crystallization of the recording layer is generally required and such crystallization is accomplished by a process called initialization.
Various processes have been proposed for the initialization. Typical processes are the process wherein the substrate is heated to crystallization temperature of the recording layer after the deposition of the recording layer (JP-A 3131/1990); the process called "solid phase initialization" wherein the recording layer is crystallized after its deposition by laser beam irradiation (JP-A 366424/1992, 201734/1990, and 76027/1991); a process wherein the substrate having the recording layer deposited thereon is irradiated with a flash light to utilize optical properties of the chalcogen compounds for pseudo-crystallization of the recording layer by photodarkening (JP-A 281219/1992); a process wherein the recording layer is crystallized by means of RF induction heating; a process wherein the substrate is heated simultaneously with the deposition of the recording layer for crystallization (JP-A 98847/1990); a process wherein a dielectric layer is formed as the first layer, and the recording layer is formed on the first layer and heated for crystallization, and the second dielectric layer is formed on the crystallized recording layer (JP-A 5246/1990).
Initialization by laser beam irradiation, however, is a time-consuming process and this process also suffer from insufficient productivity. On the other hand, the process involving the heating of the entire medium prohibited use of inexpensive resin substrates, since the heating during the initialization resulted in deformation of the resin substrate to result in tracking problems. Use of flash light required repeated irradiation to accomplish the crystallization, and productivity was also insufficient.
In view of such situation, an apparatus called "bulk eraser" is currently used for the initialization in commercial scale production. A bulk eraser is an apparatus which is capable of irradiating a high power gas laser or semiconductor laser beam without tight focusing so that multiple tracks can be crystallized at once. Use of such bulk eraser enables localized heating of the recording layer, and temperature elevation of the substrate is thus avoided to enable the use of a resin substrate of low heat resistance.
Initialization with a bulk eraser of a 12 cm optical recording disc, however, usually takes about several ten seconds to several minutes and the process of initialization has been the rate-determining step in the production of the optical recording disc.
In addition, conventional recording media of phase change type required repeated overwriting for a number of times before stable erasability is attained after the initialization. The properties have been generally evaluated after repeating the overwriting operations for about 10 times. Such unstable erasability is estimated to be the result of incomplete formation of the AgSbTe.sub.2 crystalline phase and In-Te crystalline phase.
In order to eliminate the step of initialization which had been necessary in the production of the phase change type recording medium, the inventors of the present invention proposed in Japanese Patent Application No. 47822/95 formation of the In-Ag-Te-Sb-based recording layer in separate steps of sputtering Sb plus In and sputtering Ag plus Te; or alternatively, in separate steps of sputtering Sb, sputtering In, and sputtering Ag plus Te. The recording layer formed by such step is at least partially crystallized. The recording layer formed by such process exhibits change in reflectance as in the case of initialization by bulk eraser as described above after full diffusion and mixing of the elements in the layer by repeated recording. The erasability is unstable during the first several overwriting operations immediately after the formation of the recording layer as in the case of conventional recording medium of phase change type. More specifically, reflectance of the region crystallized during the formation of the recording layer is different from the reflectance of the region crystallized in the overwriting, and the reflectance are not stabilized until whole recording layer are overwritten. In the mark edge recording used in rewritable digital video discs (DVD-RAM) and the like, such variation in the reflectance may be erroneously recognized as mark edge.
JP-A 106647/96 discloses a phase change type recording medium having disposed thereon a recording layer of an AbInSbTe-based synthetic lattice film wherein AgSbTe.sub.2 film and In-Sb film are disposed one on another, or AgSbTe.sub.2 film, In film and Sb film are disposed one on another. One merit of JP-A 106647/96 is reduction in initialization energy of the whole recording layer owing to the use of the crystallized AgSbTe.sub.2 film.
However, it has been confirmed by the inventors of the present invention that, when the AgSbTe.sub.2 film and the In-Sb film are disposed, the reflectance is not stabilized in the first several overwriting operations after the formation of the recording layer as in the case of the Japanese Patent Application No. 47822/95, and also, that the reflectance is not stabilized in the first several overwriting operations when the AgSbTe.sub.2 film, the In film and the Sb film are disposed. To obtain stable reflectance in the crystalline portion upon the overwriting, presence of In-Te crystalline phase in the crystalline portion is necessary. However, when the In is not present in the AgSbTe.sub.2 film but present as In-Sb film or In film as in the case of JP-A 106647/96, formation of the In-Te crystalline phase through the binding of In with Te is difficult. In addition, when the initialization is effected with a low energy as in the case of JP-A 106647/96, In-Te crystalline phase is not fully formed and the reflectance is not stabilized until In-Te crystalline phase is fully formed after repeating the overwriting operations for several times. It should be noted that JP-A 106647/96 is silent about specific conditions of the initialization (e.g. linear velocity and laser power).
Furthermore, JP-A 106647/96 only discloses the embodiments wherein the Sb film or the In-Sb film has a thickness of up to 5 nm. The Sb and the In-Sb films formed will not be crystalline when the film has such thickness, and the reflectance of the as formed recording layer is significantly low. When the reflectance is low, focusing of the laser beam is difficult, and uniform heating and hence, uniform initialization is difficult.
It should be noted that In content in the In-Sb film is not disclosed in JP-A 106647/96. However, in the Examples of JP-A 106647/96, the multiple layer structure wherein the In and the Sb are present in separate In film and Sb film and the single layer structure wherein the In and the Sb are present in the In-Sb film are simply compared. Therefore, the composition of the In-Sb film is estimated to be the same as the composition of In film plus Sb film. Since the In film and the Sb film has the same thickness, In content in the In-Sb film is estimated to be about 10 to 15 at %. When the In content as high as 10 to 15 at %, formation of the crystalline film is difficult even if the film thickness were increased. The recording medium formed is also associated with the problem as described above in the initialization.
In view of the situation as described above, the inventors of the present invention proposed in Japanese Patent Application No. 352298/96 a recording layer comprising an Sb-based thin film formed from an Sb-based material and a reactive thin film formed from In-Ag-Te-based material. When the Sb is deposited independently from other elements, Sb which has a high crystallization rate substantially crystallizes during the film formation. In the optical recording media, reflectance R.sub.O of the as formed recording layer (before the initialization); reflectance R.sub.C of crystalline region (erased region) of the recording layer after repeated recording, and minimum reflectance R.sub.A of amorphous region (record mark) of the recording layer after repeated recording are generally in the relation: EQU R.sub.A &lt;R.sub.O &lt;R.sub.C
In other words, the reflectance R.sub.O of the recording layer as formed is generally lower than the reflectance R.sub.C. In Japanese Patent Application No. 352298/96, however, R.sub.O is relatively high owing to the Sb thin film which is already crystallized upon completion of the formation. For example, R.sub.O is at least about 60% of R.sub.C, and if desired, R.sub.O may be increased to the level equivalent to R.sub.C (R.sub.O =R.sub.C). The initialization is thereby facilitated, and in some cases, the initialization can be even eliminated.
The recording medium with the In-Ag-Te-Sb-based recording layer formed by conventional method can no longer substantially endure repeated overwriting after repeated overwriting operations for about 1000 times because of the decrease in the erasability. Increase in the number of overwritable operations is also desirable for the optical recording medium of Japanese Patent Application No. 352298/96.